Shark Cat 7m w/ twin F150 Yammy's

[fishingrod]

Guys, my local Volunteer Rescue Group has just repowered their 1988 7m Shark Cat.


It did have 175hp 2 Stroke Yammy's on it (v6 saltwaters non counter rotating). Thirsty, but was very quick and a real buzz to drive.


It's just been repowered with a pair of counter rotating 2015 Yammy F150s. Having Skippered it before and after, I'm disappointed at the speed the new motors are achieving.


New F150 Donks on GPS :
3800 = 19-20 knots
4000 = 22.5 to 23 knots
4600 = 27 knots
WOT 6100 = 35-36 knots.
This is with 2 POB, 250L of fuel and clean antifoul. Motors are 50hrs old. All underfloor compartments confirmed dry.


Overall, the old 175hp's were at least 10 knots faster in the cruising rev range of 3500-4600 (tel:3500-4600). Plus the 175's would easily give 45-48 knots at 5800'ish WOT. Over 50 knots (GPS) was possible even with full tanks.


I'm unsure if engine height has anything to do with it yet. The guy that fitted the new Donks said he wasn't sure and just put them at the same height as the old ones. As its moored, I won't know the cav plate height until it's next slipped.


I'm in the process of trying to identify the prop sizes of both the old and new motors. As I realize this will be a key factor. I suspect the new props are smaller. However swinging smaller props potentially means the new Donk's will work less hard during heavy tow's, maybe helping engine longevity and possibly saving some fuel when towing.
But since traveling on the plane from A to B accounts for 60-80% of the boats hours. Its probably going too slow and thus we're getting less than ideal MPG.


Any suggestions as to what speeds vs RPM we should expect from the F150's ?


If the new props do prove too small, what are your thoughts on keeping them so the engines work less hard during tows ?


Cheers
Rod.

[rexaway123]

I have twin 150 yam on my pretty new 2300 Noosacat which would be built lighter than your old one but I can easily get 41 kts at 5700 swinging 4 blades

[gofishin]

Although you have to keep in mind that you've gone from two lots of V6 175hp 2 smoke grunt to 4cyl 150hp 4 strokes, and the F150's are a strong motor, something doesn't sound quite right with the difference in numbers you have quoted.

Motor height and (mostly) props will be the answer, but don't expect to match the old figures. Are the existing props black or shiny stainless? Talk to other Rescue mobs, there must be lots of 7m sharky's with these donks, & their props/testing will help. Also ask SOLAS for some options.
Cheers
Brendon


Sent from my iPhone using Tapatalk

[Noelm]

not too sure why you want to put it on the slip to check engine height, just go for a run and look over the back to see where the cav plate is running and go from there. Is 6,100RPM all the motor will rev to, or is it where the rev limiter is cutting in? get the dealer to sort it out for you.

[Chris Tucker]

I'm pretty sure that 6100 is the rev limiter on the Yamaha (a 30 second phone call to the dealer should confirm this), suggesting your props are too small. Having said that you are going to take a bath dropping 15% in power and even more in torque but it shouldn't cost you 10+ knots.

[Duckfish]

Don't forget that it is probably propped for towing. I also lost speed from repowering a 175hp 2 stroke to 150 Four but I have gained less fumes, quieter motor, better fuel economy and better allround performance at cruising speeds. My two bobs worth.
Duck

[PROS]

%15 less power there,
less holeshot
new motors are heavier probably

reduction in performance is expected.

play with props and see how it works

[fishingrod]

Thanks all for the comments.
You all confirmed roughly what I was thinking, but I had probably under estimated the difference the drop in cylinders and torque would have made. In the scheme of things is not just 50hp down.

With Rexaway, I was thinking 40knots was closer to what I was expecting.
Have not been back onboard yet to looking cav plates on the plane. Thanks Noelm, have always done this on monos but it completely slipped my mind.

Not many groups in NSW are running the Old cats any more. Google no doubt could find some interstate.
The props are black painted stainless and I've since heard some discussion took place with the dealer in terms of sizing for towing. We certainly wouldn't want them any smaller than they are.

We all know Cat's need a bit of speed to get height and air in the tunnel working. The 20-23 knot cruise we have is probably sufficient for this. But another 2-4 knots on the cruise speed would probably give better economy.
Unfortunately they didn't order the Donks with the smart gauges with fuel flow.

cheers,
Rod

[gofishin]

... The props are black painted stainless and I've since heard some discussion took place with the dealer in terms of sizing for towing... In your/this case the towing is a very important factor, and can't be discounted.

Your black props will most likely be the std Yamaha K series. From memory they have a smaller diam than the Yam SWS2 polished SS props. They are also slightly inferior (performance wise) to the SWS2-M props I believe, but then they cost less too.

Ask your dealer to try a pair if SWS2 17M props. Bigger diam (15 1/4 from memory), should work better in your case (better cruise stats), but still allow to wind right out at WOT.

Oh, also check Yam site that the 150's can actually run the M props.

As always with props, only way to confirm theory is to try them out.
Cheers
Brendon


Sent from my iPhone using Tapatalk

[Darren Mc]

Agreed the black k series yamaha props are very ordinary. The same goes with the white alloy k series yamaha props there very ordinary as well. Basic, entry level, they work but not very well.

[Gr0wler]

I have 175hp Suzuki's on my 7m Sharkcat and with current 3 blade s/s props it tops out at 43 knots at WOT, went quicker with another set of props but laboured the motors carrying a very heavy load with 4 blokes for 2-4 night fishing trips.
As a rescue/tow vessel I believe that the 150's are under powered for doing that sought of work as a 7m Sharkcat is a heavy hull. The 200 four cyl four stroke yammies would have been a closer match to the 175 V6's